1. Field of the Invention
The present invention relates to a semiconductor memory device and a semiconductor device which is provided with the semiconductor memory device and performs non-contact communication.
2. Description of the Related Art
In recent years, an individual identification technique with the utilization of wireless communication using an electromagnetic field, a radio wave, or the like has attracted attention. In particular, an individual identification technique with the utilization of an RFID (radio frequency identification) tag as a semiconductor device which performs data communication by wireless communication has attracted attention. An REID tag (hereinafter, simply referred to as RFID) is also referred to as an IC (integrated circuit) tag, an RF tag, a wireless tag, an electronic tag, an IC chip, a wireless chip, a transponder, or a data carrier. An individual identification technique with the utilization of RFID has been useful for production, management, and the like of an individual object, and has been expected to be applied to personal authentication.
A memory is used in RFID in order to store data, ID, or a communication state. In addition, in a case where a CPU is mounted on RFID, a memory is mounted on the RFID in order to store a program or a processing variable. As a memory, a ROM (read only memory), an EEPROM (electrically erasable programmable read only memory), or an SRAM (static random access memory) is used (e.g., Non-Patent Document 1: Klaus Finkenzeller, trans. SOFEL Research and Development, RFID Handbook Second Edition, Principles and Applications of Non-contact IC Cards, (The Nikkan Kogyo Shimbun, Ltd.), pp. 243-271).
A control circuit included in RFID generally includes a sequential circuit, and a flip-flop circuit included in the sequential circuit has the same structure as an SRAM.
RFID can be classified into two types depending on whether a power supply is incorporated in the RFID or power supply voltage is supplied from a reader (also referred to as a reader/writer or a transmitter): an active-type RFID capable of transmitting electromagnetic waves including information, and a passive-type RFID which is driven with the utilization of electric power of radio waves or electromagnetic waves (carrier waves) from the reader. Of these, the active-type RFID has a structure in which a power supply for driving the RFID is incorporated and a battery is provided as the power supply. In addition, the passive-type RFID has a structure in which power supply voltage for driving the RFID is generated with the utilization of electric power of electromagnetic waves (carrier waves) from the reader, so that a structure without a battery is realized.
In a case where an SRAM is mounted on RFID as a memory, an active-type RFID mounted with a power supply is needed in order to store data in an SRAM while electric power is not supplied from a reader. However, in a case of the active-type RFID, there has been a problem in that a battery has been exhausted with time corresponding to transmittance and reception of individual information and setting of intensity of radio waves needed for transmittance and reception, and electric power needed for transmittance and reception of individual information has not been able to be generated eventually. Therefore, there has been a problem in that check of remaining capacity and change of a battery have been needed in order to keep using a semiconductor device including an active-type RFID provided with a battery for driving. In addition, in a case where RFID is mounted with a battery for driving, reduction in size of the RFID could become difficult or usage of the RFID could be restricted depending on resistance-to-temperature characteristics of the battery.